Off-TopicHelp identifying part

Right, someone will know what this is:
It's off the bottom of a cylinder on one of my machines. I'm pretty sure it's faulty. I know the RFU 1/4 is an in-line flow control valve but what is the blue part called? I think it's some sort of blocker to check the cylinders decent, but that doesn't get me far with Google, I could do with a better description before giving up and having my pants pulled down by the manufacturer.